Light scattering studies of linear polyethylene

Summary The molecular weight distribution of polyethylene single crystals at different annealing temperatures has been studied at two wavelengths of 436 and 546 nm A gradual increase in the molecular weight distribution with annealing temperatures has been observed. Also root mean-square radii of gyration increase with molecular weight while second virial coefficient decreases with increasing molecular weight.

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Zusammenfassung Von Polyäthylen-Einkristallen wurde die Molekulargewichts-Verteilung bei verschiedenen Temper-Temperaturen bei den Wellenlängen 436 und 546 nm untersucht. Es wurde ein stufenweiser Anstieg der Molekulargewichtsverteilung mit wachsender Temper-Temperatur beobachtet. Auch die Quadratwurzel der mittleren Trägheitsradien steigen mit dem Molekulargewicht an, während der 2. Virialkoeffizient abfällt.

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With 2 figures and 2 tables     

Light scattering studies of narrow fractions of polyethylene

Small-angle light scattering experiments were conducted on thin films of linear polyethylene fractions over a very wide range of molecular weights. Spherulitic structures were found in all samples with molecular weight 9 × 10⁵ or less. A rodlike morphology predominates for molecular weights between 1 × 10⁶ and 2 × 10⁶. Still higher fractions yield a very disordered superstructure. These results can be correlated with previous studies of the overall crystallization rates and the resulting properties.     

Molecular association in polyethylene glycol II. Light scattering studies

Summary The present light scattering measurements reaffirm the fact that the polyethylene glycol molecules are associated in aqueous solution through the formation of both intra- and inter-molecular hydrogen bonds. The rupture of such association by the addition of either urea or thiourea is also noticed.

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Zusammenfassung Die hier vorliegenden Lichtstreuungsmessungen bekräftigen die Tatsache, daß Polyäthylenglykol-Moleküle in wäßrigen Lösungen durch Bildung von intraund inter-molekularen Wasserstoffbrücken assoziiert sind. Durch Hinzufügen von Harnstoff und Thioharnstoff brechen diese Brücken auf.

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With 1 figure and 1 table     

Fractionation in mixtures of polyethylene oxide fractions during crystallization

Two narrow-molecular-weight-fraction polyethylene oxides were mixed and studied by differential scanning calorimetry (DSC), small-angle x-ray scattering (SAXR), microscopy, and small-angle light scattering (SALS). DSC measurements indicate the presence of two melting points at each composition, leading to a eutectic diagram with a eutectic point located close to the axis of the constituent with the lower molecular weight. The depression in melting point of the higher-molecular-weight component allows the calculation of a thermodynamic interaction parameter of ?0.09 between the two fractions, indicating that they are miscible in the melt despite the fractionation process occurring during crystallization. Two SAXR long periods are also observed at each composition, indicating phase separation during crystallization. These two phases are included into large spherulites, the structure of which depends mainly upon the crystallization characteristics of the higher-molecular-weight component. The other component must then crystallize between the spherulite lamellae. Finally, small spherulites of the order of the micron are observed from the SALS patterns inside the large spherulites (of the order of one millimeter as seen by microscopy). The two polyethylene oxide fractions give rise to different size SALS spherulites and both types of spherulites are present in the mixtures.     

Conformation of linear polyethylene in 1-chloronaphthalene: Light scattering characterization of polymers. IV

Light scattering measurements were carried out on a linear polyethylene sample NBS 1475 in 1-chloronaphthalene at 135 and 115°C to determine the weight-average molecular weight, the second virial coefficient A₂, and the z-average mean-square radius of gyration. By use of these results, the system is analyzed in terms of the interpenetration function Ψ for A₂. Observed values of A₂ are rather large but the excluded volume is nevertheless relatively small. Such behavior seem to be similar to that of semiflexible polymers. The characteristic ratio C_n,LS as determined by light scattering is found to be almost twice the literature value of 6.7, which was obtained from viscosity measurements. This discrepancy is explained by comparing the theoretical value of the Flory viscosity parameter Φ at the nondraining limit with values calculated from the light scattering results.     

Viscoelastic behavior of linear polyethylene and of paraffin mixtures

Summary The complex shear modulus of a linear polyethylene (Marlex 50), of which samples were either crystallized from the melt at various temperatures or precipitated from a dilute xylene solution, has been measured as a function of temperature at fixed frequencies between 10⁵ and 10^–1 c/s. The three relaxation regions, , can be distinguished, and activation energies are determined from the relaxation map. The-process shows a frequency-temperature characteristic which has been divided into three parts _I, _II, _III. Further experimental evidence for the kink model, proposed in an earlier paper (reference 4) is taken from the investigation of suitable paraffin mixtures. Measurements on six n-alkane systems, each consisting of an even n-paraffin mixed with a few percent of a paraffin with a chain longer by two C-atoms, are reported.

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Zusammenfassung Es wurde die TemperaturabhÄngigkeit des komplexen Schubmoduls eines linearen PolyÄthylens (Marlex 50) bei festen Frequenzen zwischen 10^–1 und 10⁵ Hz untersucht, wozu die Proben entweder aus der Schmelze bei verschiedenen Temperaturen kristallisiert oder aus verdünnter Xylollösung bei 80 C gefÄllt, in Methanol ausgewaschen und gepre\t worden waren. Drei Dispersionsgebiete, , können unterschieden werden. Die/max-1/T- Charakteristik des -Relaxations-prozesses wird in drei Bereiche _I, _II, _III unterteilt. Aktivierungsenergien werden aus geradlinigen Kurvenstücken dieser Charakteristiken bestimmt.Eine erneute experimentelle Stütze für das in einer früheren Arbeit (4) vorgeschlagene Kinkenmodell stellen Messungen an 6 weiteren n-Paraffin-Mischungssystemen dar, wobei die KettenlÄngen der Mischungspartner sich jeweils um zwei CH₂-Gruppen unterscheiden.

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Parts of this paper were read at the 4th International Congress on Rheology, Providence R. I., 1963.     

Accordion-type laser-Raman scattering by drawn linear polyethylene

One can reproduce the observed accordion-type laser-Raman (ALR) scattering of highly drawn linear polyethylene if one assums that any gauche defect in the crystal lattice which interrupts the all-trans conformation sequence of the molecular chain completely decouples the accordion-type longitudinal oscillations of the two sections on both sides of the defect. Each oscillates independently of the rest. The length of the section, smaller than the full length of the straight chain between the crystal surfaces, determines the frequency of the ALR absorption. One such defect per five chain stems of the ideal crystal yields a straight-length distribution which agrees sufficiently well with that derived from the ALR spectrum. Small-angle x-ray scattering very generally registers the resulting decrease of the electron density of the crystalline component without yielding more detailed information about the location and frequency of such gauche defects.     

Inelastic neutron scattering studies of polyethylene and N-alkanes

High quality inelastic neutron scattering (INS) spectra of randomly oriented polycrystalline polyethylene, perdeuteropolyethylene and highly oriented polyethylene are presented. The instrumental resolution was significantly better than previous work and has revealed increased detail in the 0 − 600 cm⁻¹ region. For the polycrystalline sample, comparison with the best available dispersion curves shows that these qualitatively reproduce the INS spectrum, apart from the energy of the maximum in the in-plane C-C bending mode v5. For the oriented samples, comparison with calculated INS spectra show fair, but not exact, agreement with the experimental spectra.     

Light scattering studies on polyelectrolyte complexes

The Coulomb interaction between oppositely charged polyelectrolytes leads to spontaneous formation of interpolymer complexes. Such complexes are of high practical relevance, but also interesting objects of fundamental research. The level of aggregation depends on the nature of the components as well as on the medium and external conditions of the polyelectrolyte complex formation. An appropriate combination of methods provides detailed information on the stoichiometry and structure of such systems. Especially, light scattering techniques proved to be a powerful tool in studying the structure and behavior of the complexes in solution. Selected examples will demonstrate the efficiency of such investigations.     

Electron microscope study of low molecular weight fractions of linear polyethylene

Electron microscope studies are reported for crystals of linear polyethylene formed in dilute solution from very sharp low molecular weight fractions. Emphasis is placed on molecular weights in the range of 1.1 × 10³ to 15.1 × 10³. The dependence of the crystal habit on the crystallization temperature is very similar to that which has been found for the higher molecular weight species. However, the demarcation temperature for the crystallization of the different morphological forms is very molecular weight-dependent. The conditions under which interfacial dislocation networks form can be clearly defined. The molecular weight must be less than 3000, so that these structures are restricted to very small chain lengths. However, not all crystallization conditions within this allowable molecular weight range yield such dislocations. The formation of interfacial dislocation networks are shown to occur only under very special circumstances. Their occurrence clearly cannot be offered as evidence, as has been done in the past, for a regular, chain-folded interfacial structure.     

Broad line NMR studies of linear and branched polyethylene

Temperature dependence of the shape and linewidth of the broad-line NMR spectra of commercially available high-density polyethylene (PE/HD), low-density polyethylene (PE/LD with ～3 per cent CH₃), block-copolythene (PE/AA with ～3 per cent acrylic acid) and polyethylene single crystal (PE/SC) were investigated to obtain information on the effect of branching and structural changes on the glass-transition temperature (Tg) and activation energy of molecular motion (E). The relatively lower value of Tg ～- ?100° for PE/HD compared to Tg ～- ?85° and ?60° for PE/CH₃ and PE/AA, along with the estimated lower value of E ～- 2·1 kcal/mole for PE/HD compared to 2·6 and 5·1 kcal/mole for PE/CH₃ and PE/AA, respectively, were interpreted by a molecular reorientational process connected mainly with the sidegroups of the main polymeric chains.     

Dynamic scattering properties of mixtures of cyclic and linear homopolymers and copolymers

The dynamic scattering properties of mixtures of cyclic copolymers and homopolymers are discussed. These properties are compared with those characterizing linear chain mixtures in similar conditions. The differences between cyclic and linear chains are introduced through the form factors only. The interaction parameter between different monomer species are assumed to be the same whether they belong to cyclic or linear chains. The dynamical model is based upon a generalization of the random phase approximation neglecting hydrodynamic interaction and mode coupling effects. Despite these simplifications, substantial differences are found in the dynamics of mixtures containing cyclic copolymers and homopolymers when compared to those of linear chain systems in similar conditions of temperature, concentration, and molecular weight. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1191–1200, 1997     

Small-angle x-ray scattering studies of isothermally crystallized bulk polyethylene

A set of isothermally melt-crystallized polyethylene samples was examined using small-angle x-ray scattering (SAXS). Time and temperature of crystallization were the variable parameters used to create the set of samples. Following background subtraction, desmearing, and application of the Lorentz factor to the raw SAXS data it is possible to see many orders of reflection. This suggests that much higher degrees of order are present in isothermally melt-crystallized samples than had previously been thought possible. A combination of SAXS and DSC data indicates that there is no evidence for isothermal thickening in these samples. This study, coupled with data obtained from PE single crystals, produced information concerning the extrapolation of single-crystal data to fit bulk systems. In addition, the equilibrium melting point T determined is somewhat lower than previously claimed. This study also suggests that the surface energy of the mature crystals is always lower than that of the nucleated state and/or the nucleation factor Kσ_en increases with decreasing supercooling.     

Light scattering studies on solutions of chlorinated rubber

Summary Measurements have been made of the depolarisation factors _u , _v , _h, and the intensity of scattering in the horizontal transverse direction, in the case of solutions of four different samples of chlorinated rubber in carbon tetrachloride. The size, shape and molecular weight of the micelles have been deduced by the application of the light scattering theories of Gans, Vrklajan and Katalinic and Debye. The extent to which the degradation of the rubber molecule occurs on chlorination has also been assessed.

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Zusammenfassung Für Lösungen von vier verschiedenen Proben von chloriertem Kautschuk in Tetrachlorkohlenstoff wurden Messungen der Depolarisation und der Lichtstreuung durchgeführt. Größe, Gestalt und Molekulargewicht wurden unter Verwendung der Theorien der Lichtstreuung von Gans, Vrklajan, Katalinic und Debye abgeleitet. Die Stärke des Abbaues, dem die Kautschukmoleküle bei der Chlorierung unterworfen sind, wurde abgeschätzt.

     

Wide-angle neutron scattering studies of mixed crystals of polyethylene and deuteropolyethylene

Wide-angle neutron scattering studies have been made of a series of mixed crystal blends of polyethylene and deuterated polyethylene in the range 0.68 ≤ K = 4πλ^?1 sinθ ≤ 2.02 Å^?1. Recent calculations by Stamm have shown that regularly folded subunits of small numbers (two to ten) of stems should produce an observable modulation of the diffuse scattering pattern of such mixtures in this K range. For both melt- and solution-crystallized blends, no extra signal was observed above the background scattering level. The data are used to place an upper limit (of about four) on the number of stems that could be regularly folded in the (100) and (010) planes for these materials.     

Light scattering and image analysis studies in polymer blends

Various optical techniques have been investigated as potential candidates for characterization of multiphase polymeric materials. The model calculations and corresponding experiments (time‐resolved light scattering and image analysis) have been conducted to investigate the kinetics of phase dissolution of polymer blends. The blends studied were polystyrene/poly (methyl methacrylate) mixtures with diblock copolymer composed of the corresponding homopolymers. The time evolution of the spinodal peak position q_m(t,T) and the scattered intensity maximum I_m(t,T) at q_m have been compared with theoretically predicted values of exponents for distinct time scales of the phase dissolution in various temperature regimes.     

Morphology of melt-crystallized linear polyethylene fractions and its dependence on molecular weight and crystallization temperature

Replicas and thin-section electron microscopic studies were made of fractions of linear polyethylene covering the molecular weight range 2.78 × 10⁴ to 6.0 × 10⁶ for a variety of crystallizing conditions. Lamellar crystallites were found under all circumstances; and the supermolecular structure, or crystalline morphology, is in agreement with that previously reported from an analysis of the small-angle light-scattering patterns of the same samples under similar crystallization conditions. Details of the crystalline microstructure are also described, which range from truncated hollow pyramids which degenerate as the molecular weight or the undercooling are increased. From these results, it is possible to describe the mechanism of formation of polyethylene spherulites.     

Light scattering and dielectric relaxation studies of glass forming liquids

Extended Abstract: Glass forming organic liquids and polymers exhibit long range density fluctuations with correlation length ξ in the range of 10–300 nm at temperatures above T_g (1 - 6). This follows from dynamic and static light scattering experiments revealing some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (i) In static light scattering the intensity I(q → 0) is no longer proportional to the isothermal compressibility, (ii) This excess scattering I_exc shows a strong q-dependence (q = (4π/Λ.)sin(θ/2)) corresponding to a correlation length ξ in the above mentioned range, (iii) The Landau-Placzek ratio I_Rayleigh/2I_Brillouin is much too high compared with the results of light scattering theories, (iv) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (Γ ˜ q²) is detected with relaxation rates Γ about 10⁻⁶ to 10⁻⁹ lower than those of the α-process at a given temperature. In order to explain these observations, a two-state fluid model is proposed, which starts from the coexistence of “liquid-like” and “aperiodic solid-like” regions within the liquids. Such ideas have been discussed many times before, so for example A.R. Ubbelohde (7) speculates about “anticrystalline” clusters in liquids. Molecular dynamics simulations of atomic liquids showed that long range orientational fluctuations appear upon supercooling (8). A preferred icosahedral ordering is observed (9) and the number of icosahedral clusters increases with decreasing temperature (10). In connection with the interpretation of the dynamics of supercooled liquids different “two-state” models have been proposed (11 - 15). For the explanation of the light scattering results we propose that the molecules in the different dynamic states (“liquid” or “solid”) aggregate during annealing of the liquid at temperatures above T_g. Experiments showed that the equilibration times can be rather long (3 - 5), but nevertheless the liquids exhibiting long range density fluctuations are in the state of lowest free energy. We claim that our observations are the first experimental proof of the existence of such different dynamic states, which have been discussed many times before. The extended secondary clusters can also be detected by ultra small angle X-ray scattering.